1. Field of the Invention
This invention relates to digital signal processing (DSP) techniques for analysing the frequency content of a signal.
2. Description of the Prior Art
The most common known method employed to perform frequency analysis is the FFT (Fast Fourier Transform) method which has embodiments in both software algorithms and in hardware form. The FFT approach suffers from the disadvantage that it is better suited to software implementations rather than hardware and that it is difficult to realise a flat frequency response across a “bin” whilst maintaining good cut-off of adjacent frequency “bins”.
A multistage frequency separating circuit for use in frequency separating a simple signal is described in British Published Patent Application GB-A-2 258 963. There are significant differences, however, between this circuit and the present invention. The above circuit uses successive stages of low-pass and high-pass filters to separate the signal into a number of frequency bands whereas the present invention uses successive stages of complex up and down-converters, each stage being centered at zero frequency. This has the advantage of greatly simplifying the filters which are now all low-pass filters of virtually identical design. This, in turn, leads to further simplification by the use of massive interleaving of different data streams through a common interleaving filter. It also means that all output data for each frequency “bin” is based on zero centre frequency (which is similar to the FFT output) rather than at an offset frequency which is the case for the above circuit.